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Measurements of the gas mass are necessary to determine the planet formationpotential of protoplanetary disks. Observations of rare CO isotopologues aretypically used to determine disk gas masses; however, if the line emission isoptically thick this will result in an underestimated disk mass. With ALMA wehave detected the rarest stable CO isotopologue, 13C17O, in a protoplanetarydisk for the first time. We compare our observations with the existingdetections of 12CO, 13CO, C18O and C17O in the HD163296 disk. Radiativetransfer modelling using a previously benchmarked model, and assuminginterstellar isotopic abundances, significantly underestimates the integratedintensity of the 13C17O J=3-2 line. Reconciliation between the observations andthe model requires a global increase in CO gas mass by a factor of 3.5. This isa factor of 2-6 larger than previous gas mass estimates using C18O. We findthat C18O emission is optically thick within the CO snow line, while the 13C17Oemission is optically thin and is thus a robust tracer of the bulk disk CO gasmass.

The First Detection of 13C17O in a Protoplanetary Disk: a Robust Tracer of Disk Gas Mass